The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a minimum transmission ratio.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is determined by line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio. In such a system, the speed of changing of the transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions.
Accordingly, hunting or overshooting of the transmission ratio occurs, which reduces the driveability of the vehicle.
European Patent Application No. 205257 discloses a transmission ratio control system in which the desired transmission ratio for obtaining a transmission ratio changing speed is derived from a look-up table having an X-axis for opening degree (.theta.) of a throttle valve of an engine and a Y-axis for vehicle speed represented by driven pulley speed (N.sub.S). Such a table has a very large capacity because the vehicle speed must be divided into a large number of grades. In addition, the table does not provide data for operation at transient states of the transmission, such as the kickdown, because the opening degree (.theta.) is at an extreme value in the transient state.